Lyme disease is the most common vector-borne disease in the United States and the number of reported cases has doubled to over 20,000 per year during the past decade. Lyme disease is caused by the bite of a tick infected with the spirochete, Borrelia burgdorferi, the etiologic agent of the disease. Evaluation of the potential for development of disease from a tick bite is complicated by incomplete knowledge of the diversity of spirochete genotypes in nature and their pathogenic capacity. Our previous studies have demonstrated that different genotypes of B. burgdorferi s. s. possess varying potential for dissemination in an infected host. In the current application we propose to extend and build upon these findings with the following specific aims: 1) Molecular typing of B. burgdorferi in ticks and wildlife will be pursued in order to elucidate the population structure of B. burgdorferi genotypes in ticks and wildlife reservoir hosts and address how differences in B. burgdorferi genotype distribution between reservoir hosts contribute to maintenance of pathogenic genotypes in nature and their transmission to humans; 2) dissemination, acquisition and transmission of B. burgdorferi of different genotypes from and to ticks will be assessed in a murine model; 3) the role of outer surface protein C (OspC) in virulence and dissemination will be elucidated; and 4) the relationship between B. burgdorferi genotype, the quantity of spirochetes in blood of patients and clinical features of Lyme disease will be assessed. The proposed combination of molecular, ecological and clinical approaches proposed here should further clarify the extent of B. burgdorferi genotypic diversity in nature and the role that such diversity plays in Lyme disease risk and pathogenesis.